The invention relates to a process for the production of thermal sheets and thermal sheets produced thereby. By "thermal sheets" we mean heated rear window panes for motor cars, as well as ship porthole panes, train and aircraft window panes, windshields for any kind of vehicle, and furthermore glass sheets or panes which can be used for factories, shops and houses.
As it is known, thermal sheets are made of a sheet of safety glass fitted at least on one side with electrically conductive stripes, the purpose of which is to heat the sheet itself so that demisting and/or defrosting thereof is obtained.
It is known that the aforesaid electrically conductive strips are generally produced by screen-printing of suitable paints and inks made of a frit--that is to say, a binder--and of a conductive metal, suitably mixed together so as to obtain desired electrical resistance values, and the desired power dissipation values deriving therefrom.
To vary the amount of heat in certain portions of the glass sheet, the two following processes are generally employed:
(a) Carrying-out of galvanic reinforcing--e.g. with copper--on the strips themselves, graduating the abovesaid reinforcings for thickness and location on the strips; or PA1 (b) Direct alteration of thickness or width of the strips.
In all these cases, several problems and inconveniences have to be dealt with.
Thus, the transfer of the electric circuit on the screen-printing frame is troublesome, as it is necessary to vary, as continuously and as precisely as possible, the cross-section of necessarily small lines (0.4 to 0.8 mm) on the negative slide from which (by means of a direct photographic system) the impression of the desired electrical circuit on a photosensitive jelly is obtained.
During print and subsequent stoving, at the thinnest sections or where passages from one section to another take place, circuit breakage may easily occur, due, for example, to the solvent evaporating, which leave a portion without conductive metal, thus causing rejects.
Cross-section reductions or single points may cause dissipated power peaks and, therefore, overheat and consequently cause circuit breakage during operation thereof
When, due to special aesthetical functional requirements, a concentration of filaments is necessary, this causes overheating of the portion over which this concentration occurs, unless thicker wires are used. This is aesthetically displeasing, and also dangerously reduces the pane's transparency
An imprecise graduation of the width change also causes steps that may appear displeasing to the eye
In places where resistance must reach low values, vast width increases are necessary, creating thus more unsightly areas.